Each energy meter used for billing must have a test output. An optical indication is mandatory. Beside this the meter can also have electrical test outputs.
We have a nice three phase power supply in our home, but where does it come from? And why does the waveform has a sine shape?
The interactive diagram below shows the relationship between the phase angle φ, apparent-, active- and reactive power respective energy in accordance with clauses 12 and 14 of IEC 60375.